Recently, according to commercialization of long term evolution (LTE) mobile communication terminals and various communication services such as the Internet of things, frequency bands required to be supported by one terminal are gradually increased and the antenna size becomes small due to a slim design of a product and employment of a high-capacity battery.
Under such a situation, researches for developing antennas capable of realizing multi-bands and a broadband with a small size are being performed in various aspects of a design technique and manufacturing process methods, but these can't overcome the size limitation of the antenna. In particular, for an LTE communication terminal, in order to overcome the difficulty in realizing broadband antenna characteristics, frequency band switching using a tunable antenna module or an RF switch such as SPDT is applied thereto, but there are disadvantages in cost and complexity of circuit design.
For example, FIG. 1A illustrates a method for controlling an LC value inside a TAM to tune antenna matching in real time by determining a traveling wave toward an antenna and a reflection wave from the antenna respectively through power detection and by digitally adjusting a DAC value in order to maintain a relative reflection amount smaller than a certain reference value (i.e. to manage on the basis of a voltage standing wave ratio (VSWR)).
FIG. 1B illustrates a structure for controlling grounding and feeding terminal positions of an antenna to switch it to a desired frequency. In the drawing, SW1 and SW2 denote switches, and M1 and M2 denote matching circuits.
This structure uses a resonant frequency shift according to a difference between resonant lengths of an antenna when the SW1 is connected and when the SW2 is connected.
According to a typical technique illustrated in FIG. 1A, there is a limitation in that a software algorithm for optimizing performance is complicated, a manufacturing cost increases due to the application of the TAM, a complex control circuit is necessary for controlling the TAM, and it causes a lack of PCB mounting area. In addition, since highly lossy L and C are applied to broaden the tuning range, loss by the lumped elements grows. Furthermore, application of external DC power may cause a noise issue on the antenna.
According to a typical technique illustrated in FIG. 1B, a degree of frequency shift varies according to an adequate separation distance d between the grounding and feeding lines, and when a distance from the feeding terminal is out of a certain distance, an antenna matching characteristic becomes worse in a specific frequency band. Accordingly, when a large amount of frequency shift is necessary, a characteristic of an unselected frequency band according to on/off of the switch becomes degraded. In addition, since an antenna element and a DC power line are electrically connected, sensitivity of a received signal may be degraded by an influence of antenna noise due to the power line.